Chemistry Laboratory Report --- Classification of Chemical Substances

Results

• The metals are: aluminum, zinc, tin, magnesium, iron
• The molecular substances are: iodine, sucrose, glucose, naphthalin and powdered cellulose
• The ionic substances are: sodium chloride, potassium permanganate, potassium dichromate, cupric sulfate, calcium hydroxide, cobalt(II) chloride hexahydrate,  sodium acetate and ammonium chloride.
• The macromolecular substances are: graphite, starch

Explanation

• Metallic Substances

•  Metals are chemical elements whose atoms readily lose electrons to form positive ions (cations), and form metallic bonds between other metal atoms and ionic bonds between nonmetal atoms. Its structure is described as a lattice of positive ions surrounded by a cloud of delocalized electrons.
• Metal’s appearance is often shiny, silvery and grey. This luster property is due the “sea of electrons” structure of metals. The free-moving electrons thus interact with the light photons, mass-less packets of pure energy that can interact with the orbital electrons of elements and molecules, kicking them up into higher more energetic orbits. In this case, the energy emitted is light energy. This reflection of light results in the luster property of metals.
• Metals do not dissolve in water as the metallic attraction between the cation and anion is much stronger than the dipole-dipole attraction of water, so the metals stay intact instead of dissolving. Metal is also non-polar, so it does not dissolve in water, which is polar.
• Metal is non-polar in nature, so according to the like-dissolves-like rule, it is insoluble in the polar propanol.
• Metals have relatively high melting points as there are many van der Waals’ forces between molecules, and there are strong metallic bonds between positive ions and the electron sea.
• The electrical conductivity of metals originate from the fact that in the metallic bond, the outer electrons of the metal atoms form a gas of nearly free electrons, moving as an electron gas in a background of positive charge formed by the ion cores. These free electrons allows electric conduction to occur, as they are attracted to the positive pole.

• Molecular Substances

• Molecules are made of fixed numbers of atoms joined together by covalent bonds, and can range from the very small (even down to single atoms, as in the noble gases) to the very large (as in polymers, proteins or even DNA).  All sugars and acids are molecular.
• Most molecular substances are insoluble (or only very sparingly soluble) in water. Those which do dissolve often react with the water, or else are capable of forming hydrogen bonds with the water.
• Sucrose and glucose, as exceptions, dissolve in water, because water is covalently bonded (non polar) but it has a positive and negative end region (polar). This makes it an attractive solvent to both polar (like salt) and non polar (like sugar). Sucrose and glucose are polar covalent molecules and their polarity will be attracted to the polarity of the surrounding water molecules, thus making it soluble. It has a lot of hydroxyl groups that can form hydrogen bonds with the water molecules.
• Most molecular substances dissolve in alcohol although alcohol is polar, because it dissolves covalent bonds.
• Molecular substances are often soluble in organic solvents - which are themselves molecular. Both the solute (the substance which is dissolving) and the solvent are likely to have molecules attracted to each other by van der Waals forces. Although these attractions will be disrupted when they mix, they are replaced by similar ones between the two different sorts of molecules.
• Molecular substances won't conduct electricity as it has no free electrons. Even in cases where electrons may be delocalised within a particular molecule, there isn't sufficient contact between the molecules to allow the electrons to move through the whole solid or liquid.
• Molecular substances have relatively low melting points than ionic compounds  because in molecular compounds, molecules are held by weaker van der Waals’ forces, so it needs less heat energy to change its state in compare to the stronger ionic bonds between ionic compounds.

• Ionic Substances

•  Ionic substances are chemical compounds in which ions are held together in a lattice structure by ionic bonds. Usually, the positively charged portion consists of metal cations and the negatively charged portion is a halogen or polyatomic ion. Ions in ionic compounds are held together by the electrostatic force between oppositely charged bodies.
• Many ionic solids are soluble in water - although not all. It depends on whether there are big enough attractions between the water molecules and the ions to overcome the attractions between the ions themselves. Positive ions are attracted to the lone pairs on water molecules and co-ordinate (dative covalent) bonds may form. Water molecules form hydrogen bonds with negative ions. The concept that most ionic compounds is polar, containing a dipole-dipole attraction also contribute to the fact that most of them dissolve in water.
• Ionic solids are commonly not soluble in organic solvents. The attractions between the solvent molecules and the ions aren't big enough to overcome the attractions holding the crystal together. Also, its polar nature makes it insoluble in non-polar organic solvents.
• Most ionic substances do not dissolve in alcohol as alcohol is non-polar, so ionic compounds that are polar cannot dissolve in it, according to like-dissolve-like theory.
• Cobalt (II) chloride hexahydrate dissolve in alcohol, as the water in the compound is polar, and thus it dissolves in the polar alcohol.
• Solid ionic compounds do not conduct electricity, because there are no electrons which are free to move. When it melts, ionic compounds undergo electrolysis, which involves conduction of electricity because of the movement and discharge of the ions. In the process, cation and anion are produced, the free electrons can now conduct electricity.
• The melting point of ionic compounds is relatively higher than molecular compounds as they have van der Waals’ forces as well as dipole-dipole attraction. So it requires more energy to change its stage.

• Macromolecular Substances

• Macromolecular is a large molecule made up of a large numbers of atoms, comprised of at least one-hundred atoms. Macromolecules may also be referred to less specifically as polymers.
• It is not soluble in propanol, water or hexane as the covalent bonds in between molecules are much stronger than the attractive forces of water.
• It does not conduct electricity mostly because there is no free electrons available. But for graphite, it can conduct electricity due to the vast electron delocalization within the carbon layers. These valence electrons enable it to conduct electricity. However, the electricity is only conducted within the plane of the layers.
• The melting points of macromolecule tends to be high as there are many covalents bonds in between molecules, and thus a large amount of energy is needed to change its state

Points to Improve

• The obtained melting points of molecules varies a lot from the theoretical results
• Some of the samples cannot be melt by the Bunsen flame, which temperature is too low
• Some of the samples is insoluble, thus the result of aqueous conductivity cannot be obtained

Improvements

• Performing the experiment multiple times can increase accuracy of results
• More accurate temperature measuring instruments can be used.
• A flame of higher temperature can e used to melt the samples and find the actual melting point.